As far as 2 horses for your own use are concerned, there shouldn't be a problem.
Grazing horses is, for planning purposes, classed as agricultural. For that reason, there also shouldn't be a problem with using the outbuldings to house them. The fact that you are planning on making hay - presumably to feed them with makes your plans totally feasible and allowed
Where horses do become a planning issue is where you may have a livery, and the use of land is then a business, which is not basically agricultural, and you would then technically have to apply for change of use. You would also need planning for a manege.
As far as planning is concerned, there are actually 6 different categories for the keeping of horses. many planners do not know them all and I have stated the simplest ones. But when I said "technically" you will often find that a few horses on land, even rented out to others, for example is not normally flagged up.
What does get noted is the erection (without planning) of a big new stable block, tack room, static caravan for rest room, and maybe a manege. That shouts out "business use" and would require a change of use. Your plans to make hay, presumably to feed them with, further reinforces the agricultural nature of your horses.